The present invention relates to a process and apparatus for smelting a material containing noble metals, especially an anode slime produced in connection with the electrolytic copper process, and for recovering the noble metals present therein, whereby slag-forming materials are mixed with the material containing noble metals, whereafter the mixture is smelted to produce a metal and a slag phase; the slag is then removed at least partly before the melt is exposed to an oxidizing treatment, and finally the refined metal is recovered.
So far, the Dore furnace has been used for the above purpose. It is a manually operated, and therefore relatively small, reverberatory with a flat floor and works according to the batch principle. This type of furnace has a relatively low furnace chamber, the wall of which is provided with a mouth for shoveling anode slime and slag-forming materials into the furnace chamber and for letting the melt and the slag out of the furnace. The lower part of the furnace mouth is first bricked up and then anode slime and slag-forming materials are shoveled into the furnace. Soda and borax are generally used as slag-forming chemicals.
The furnace chamber wall also has burners and air channels for heating the contents of the furnace chamber, and its ceiling has an outlet for the smoke gases. To provide an even heating, the burner flames are directed obliquely downwards towards the melt and the slag on top of it. The aim is to produce a reducing atmosphere in the furnace chamber, whereby metal is separated in small drops from the slag phase and falls onto the floor of the furnace chamber, forming a metal phase there. The temperature is kept at approx. 1200.degree. C. during the smelting and the phase separation, the aim being to bring the slag into an amorphous state.
To let the slag out of the furnace, a narrow groove is chipped or drilled in the upper part of the brickwork in the furnace mouth, the groove extending to the upper surface of the melt. The letting out of the slag is discontinued when the slag layer is thin enough, i.e., when metal drops can be seen in the following slag. Thereafter the rest of the brickwork is taken down and the rest of the slag is removed manually with scraper.
Thereafter follows the oxidizing or refining stage of the metal melt in the furnace chamber, whereby the furnace atmosphere is controlled so that it is an oxidizing one, and air or oxygen is fed through blow pipes onto the surface of the melt. The blowing is directed diagonally along the metal surface. Even at this stage, slag is removed by means of a scraper and by using cement for cementing the slag. The temperature is approx. 1100.degree. C.
The smelting takes approx. 2 days in total and is preferably carried out to such an extent that even lead is obtained in the metal phase. It has been noted that lead drops falling towards the furnace floor effectively wash the noble metals from the slag phase.
The oxidizing stage then lasts one more day. Thereby the metals which oxidize easier are oxidized first and are transferred into the slag phase, which is then separated from the metal phase. The latter is finally cast in molds. Such easily oxidizing metals which are removed along with the slag are primarily lead and tellurium.
This previously known reverberatory has several significant disadvantages, such as:
Large amounts of gas are produced in the furnace chamber and carry away along with them a great deal of valuable dust, which constitutes a great circulating load when it is returned from a wet-wash venturi positioned after the furnace. Most of this load accumulates in the oxidizing stage, thereby increasing the impurities in the noble metal.
The number of burners is limited and therefore their effect must be extended over as large an area as possible. The flames must be directed onto the surface of the batch, whereby a considerable difference in temperature is created between the surface and the bottom of the batch. Because the metal phase accumulating at the floor of the furnace chamber must, however, be kept molten, the surface of the relatively thick slag phase must be heated more than would otherwise be necessary, which naturally increases fuel costs and causes wear on the lining bricks of the furnace chamber. Therefore the furnace chamber is very flat to reduce the thickness of the batch itself. Thus, the contact surface between the metal melt and the furnace chamber floor is very large, whereby more than a normal amount of noble metal is caught in the pores and seams of the lining material and increases noble metal losses.
The size of the furnace is relatively large in proportion to the final amount of pure metal, which constitutes only approx. 10% of the original volume. On the other hand, the size of the furnace is limited by its manual operation, for drawing out the slag and mixing the slag-forming materials with the melt take place manually, a factor which naturally limits the dimensions of the furnace. Thus, very great amounts of refined noble metal cannot be obtained from the furnace in one batch.
The total treatment period of one batch is very long, approx. 3 days.
The work is heavy three-shift work, and the periodical operation rapidly wears out the linings.
The object of the present invention is to eliminate the above disadvantages and to provide a process and apparatus for the smelting of noble metals and for the recovery of noble metals, especially from the anode slime produced in connection with the electrolytic copper process.